Herpes simplex virus type 1 (HSV-1) causes clinically significant acute and life-long latent infections characterized by periodic recurrences in a large portion of the population worldwide. Although virus shed following reactivation from latency is recognized as the primary source of infection in spread to uninfected individuals, little is known about the molecular mechanism responsible for reactivation. The long-term goals of Project 3 are to identify and determine the roles of the viral and cellular factors involved in the establishment, maintenance and reactivation of neuronal latency. Because reactivation is the most experimentally approachable of these processes, Project 4 will address three central aspects of reactivation. In contrast to productive infection, latent viral genomes are in a chromatin-like, transcriptionally unavailable configuration such that productive phase viral gene expression is completely repressed at the time reactivation begins. In Specific Aim 1, sensitive RT-PCR and PCR procedures will be used to determine the order of viral gene expression relative to the onset of viral DNA replication to test the hypothesis that expression of viral genes involved in viral DNA replication-and viral DNA replication itself-precede expression of other classes of viral genes during reactivation in neurons. Although viral DNA replication appears to begin at both types of HSV-origin, oriL and oriS, evidence that viral DNA replication is initiated preferentially at oriL during reactivation in neurons has been obtained. Using viruses with mutations in oriL and oriS that disrupt origin function, this hypothesis in mouse TG explants, TG cell cultures and in the rabbit ocular model of reactivation in Specific Aim 2. Although neurons are terminally-differentiated and growth-arrested, we have shown that the activities of cellular cyclin-dependent kinases (cdks) are induced in reactivating neurons, required for both the transactivating activity and post-translation modification of at least one viral regulatory protein (ICPO), and required for reactivation. In Specific Aim 3 we will attempt to identify the cdk(s) involved in these activities in neurons, and determine how they alter the transactivating activity and post-translational modification of ICPO in cells of neural lineage.